Re: [PATCH 26/26] x86, pkeys: Documentation

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Sep 24, 2015 at 12:10 PM, Dave Hansen <dave@sr71.net> wrote:
> On 09/24/2015 02:49 AM, Ingo Molnar wrote:
>> * Dave Hansen <dave@sr71.net> wrote:
>>>> Another question, related to enumeration as well: I'm wondering whether
>>>> there's any way for the kernel to allocate a bit or two for its own purposes -
>>>> such as protecting crypto keys? Or is the facility fundamentally intended for
>>>> user-space use only?
>>>
>>> No, that's not possible with the current setup.
>>
>> Ok, then another question, have you considered the following usecase:
>>
>> AFAICS pkeys only affect data loads and stores. Instruction fetches are notably
>> absent from the documentation. Can you clarify that instructions can be fetched
>> and executed from PTE_READ but pkeys-all-access-disabled pags?
>
> That is my understanding.  I don't have a test for it, but I'll go make one.
>
>> If yes then this could be a significant security feature / usecase for pkeys:
>> executable sections of shared libraries and binaries could be mapped with pkey
>> access disabled. If I read the Intel documentation correctly then that should be
>> possible.
>
> Agreed.  I've even heard from some researchers who are interested in this:
>
> https://www.infsec.cs.uni-saarland.de/wp-content/uploads/sites/2/2014/10/nuernberger2014ccs_disclosure.pdf
>
>> I.e. AFAICS pkeys could be used to create true '--x' permissions for executable
>> (user-space) pages.
>
> Just remember that all of the protections are dependent on the contents
> of PKRU.  If an attacker controls the Access-Disable bit in PKRU for the
> executable-only region, you're sunk.
>
> But, that either requires being able to construct and execute arbitrary
> code *or* call existing code that sets PKRU to the desired values.
> Which, I guess, gets harder to do if all of the the wrpkru's are *in*
> the execute-only area.
>

This may mean that we want to have a way for binaries to indicate that
they want their --x segments to be loaded with a particular protection
key.  The right way to do that might be using an ELF note, and I also
want to use ELF notes to allow turning off vsyscalls, so maybe it's
time to write an ELF note parser in the kernel.

--Andy

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